Virwox has some minimum fees for various things, so if you buy a small amount of Bitcoin those fees will seem big. You can't just buy Bitcoins there either. You have to buy Linden dollars, then use those to buy Bitcoins, so you pay trading fees twice. The markets are thin and if you refuse to put a bid order on and wait for someone to fill it you will get a bad exchange rate. That's not a virwox tax, it's a result of thin markets.